Traffic Collision Avoidance Systems (TCAS) use two antennas, one on the top and one on the bottom of an aircraft, each of which is used to estimate the relative beaing between own aircraft and an intruder. Each traditional TCAS antenna has four elements, which are placed orthogonally on the same plane as shown in FIG. 1. A TCAS system interrogates the transponder on other aircraft. The transponders of other aircraft respond with a reply which may contain altitude or other information. The TCAS uses the reply signal and its multi-element directional antenna to estimate the relative bearing of the other aircraft. In some systems the phase difference of the reply signal received at elements E1 and E3 is proportional to the sine function value of the intruder's bearing angle, and the phase difference between elements E2 and E4 is proportional to cosine function value of the intruder's bearing angle. (Where E1 E3 pair and the E2 E4 pair are orthongonal). The system can estimate the bearing from the signals received on one multi-element antenna. Although this is an adequate setup for determining bearing, it is redundant, costly and comes with a weight penalty because of the two antennas and eight antenna elements needed.